Nitrous oxide is widely used clinically for conscious sedation and also in surgical anesthesia, however, the mechanism of its analgesic action has remained unknow. Other investigators have reported that nitrous oxide-induced analgesia is antagonized by the opiate receptor blocker naloxone and, in preliminary investigations, we have found that exposure to 75% nitrous oxide: 25% oxygen produces a reversible increase in the methionine-enkephalin (ME)-like immunoreactivity in fractions of artificial cerebrospinal fluid collected from ventricular-cisternally-perfused, anesthetized rats. Based upon these findings, we hypothesize that nitrous oxide can induce neuronal release of specific opioid peptides and that this drug action might be associated with the analgesic effect of nitrous oxide. Our specific aim is to submit this hypothesis to testing by a set of postulates designed to verify an opiate involvement in a physiological or pharmacological process. These tests will involve 1. antagonism by naloxone, 2. non-antagonism by (+)-naloxone, 3. cross-tolerance with morphine, 4. potentiation by inhibitors of enkephalin metabolism, and 5. demonstration of release of opioid peptides. These tests will be conducted, using nitrous oxide-induced analgesia in behavioral models and nitrous oxide-induced changes in perfusate ME-like immunoreactivity in anesthetized and conscious centrally-perfused rats. Our methods will include the rat tail flick test, the rat hot plate test, ventricular-cisternal perfusion, intrathecal perfusion, radioimmunoassay and high performance liquid chromatography separation or assay of opioid peptides. The long-term objectives of this research are to systemically analyze the drug actions and effects of nitrous oxide and eventually other general anesthetic drugs as well. The findings of this research should contribute to our understanding and knowledge of the pharmacology of nitrous oxide.